Generally, the widely-used peripheral input device of a computer system includes for example a mouse device, a keyboard device, a trackball device, or the like. Via the keyboard device, characters or symbols can be directly inputted into the computer system. As a consequence, most users and most manufacturers of input devices pay much attention to the development of keyboard devices.
A keyboard device with scissors-type connecting elements will be described as follow. FIG. 1 is a schematic cross-sectional view illustrating a conventional keyboard device. The conventional keyboard device 1 comprises plural keys 10, a membrane switch circuit member 11 and a supporting plate 12. Each key 10 comprises a keycap 101, a scissors-type connecting element 102 and an elastic element 103. In the key 10, the keycap 101 is exposed outside the conventional keyboard device 1. Consequently, the keycap 101 can be depressed by the user. The scissors-type connecting element 102 is used for connecting the keycap 101 and the supporting plate 12. The elastic element 103 is penetrated through the scissors-type connecting element 102. In addition, both ends of the elastic element 103 are contacted with the keycap 101 and the membrane switch circuit member 11, respectively. The supporting plate 12 is located under the membrane switch circuit member 11. The keycap 101, the scissors-type connecting element 102, the elastic element 103 and the membrane switch circuit member 11 are supported on the supporting plate 12.
The membrane switch circuit member 11 comprises an upper wiring board 111, a separation layer 112, and a lower wiring board 113. The upper wiring board 111 has plural upper contacts 1111. The separation layer 112 is located under the upper wiring board 111, and comprises plural perforations 1121 corresponding to the plural upper contacts 1111. The lower wiring board 113 is located under the separation layer 112, and comprises plural lower contacts 1131 corresponding to the plural upper contacts 1111. The plural lower contacts 1131 and the plural upper contacts 1111 are collectively defined as plural key switches 114. The elastic elements 103 are disposed on the membrane switch circuit member 11 and aligned with the corresponding key switches 114.
The operations of the key 10 of the conventional keyboard device 1 in response to the depressing action of the user will be illustrated as follows. Please refer to FIG. 1 again. When the keycap 101 is depressed, the keycap 101 is moved downwardly to push the scissors-type connecting element 102 in response to the depressing force. As the keycap 101 is moved downwardly relative to the supporting plate 12, the keycap 101 pushes the corresponding elastic element 103. At the same time, the elastic element 103 is subjected to deformation to push the membrane switch circuit member 11 and trigger the corresponding key switch 114 of the membrane switch circuit member 11. Consequently, the membrane switch circuit member 11 generates a corresponding key signal. When the keycap 101 is no longer depressed by the user, no external force is applied to the keycap 101 and the elastic element 103 is no longer pushed by the keycap 101. In response to the elasticity of the elastic element 103, the elastic element 103 is restored to its original shape to provide an upward elastic restoring force. Consequently, the keycap 101 is returned to its original position where it is not depressed. The structures and the operations of the conventional keyboard device have been mentioned as above.
Conventionally, the keys of the keyboard device are arranged in a keyboard matrix. When one key is depressed, a keyboard controller realizes the information of the depressed key according to the column information and the row information and outputs the corresponding key signal. Generally, the intersection between each row and each column of the keyboard matrix represents a corresponding key intersection. When one keycap is triggered, the keyboard device starts to scan the keyboard matrix and recognizes which keycap is depressed. For example, the keyboard controller scans all columns of the keyboard matrix sequentially. If the corresponding signal is received from a specified row when a specified column is scanned by the keyboard controller, the keyboard controller can realize which key is depressed according to the received column information and row information.
The minimum matrix unit of the keyboard matrix is composed of four keys. When any of the four keys is depressed, the corresponding key signal can be successfully generated. When two of the four keys are arbitrarily depressed, the keyboard controller can realize which keys are triggered according to the column information and the row information. However, when three keys are simultaneously depressed, the information of two rows and the information of two columns are transmitted to the keyboard controller. Since four keys are defined by two rows and two columns, the keyboard controller cannot recognize the three depressed keys from the four keys according to the column information and the row information only. Under this circumstance, the fourth key is erroneously judged as the on-state key. That is, the fourth key is referred as a ghost key.
For avoiding the ghosting problem, some approaches have been disclosed. In accordance with a first approach, the keyboard device is further equipped with plural diodes near the corresponding key intersections. Since the current is allowed to pass through the membrane switch circuit member in one direction through the arrangement of the diodes, the erroneous judgement of the ghosting problem is avoided. However, the approach of installing diodes near the corresponding key intersection still has some drawbacks. Firstly, the diode is not cost-effective. Consequently, the cost of the keyboard device is increased. Secondly, the plural diodes on the membrane switch circuit member increase the thickness of the membrane switch circuit member. The increased thickness of the membrane switch circuit member is detrimental to the slimness of the keyboard device. Thirdly, the process of welding the diodes at the positions near the corresponding key intersections is very complicated.
In accordance with a second approach, the keyboard device is not equipped with the keyboard matrix. The output wire of each key is connected with a microprocessor of the keyboard device. Since this approach needs a great number of output wires, it is difficult to install the output circuits. For example, in case that the keyboard device contains 144 keys, the keyboard device comprises 144 output wires. Consequently, it is difficult to install so many output circuits.
Therefore, there is a need of providing a keyboard device with low cost and capable of avoiding the ghosting problem.